thats because they aint as good a replacement for things, and dont have the capacity very well.
from what i have seen, over time , the energyser type is doing OK, and energyser at wallymart, and target is up to 850-900, and they actually ARE that capacity, the 1000's that i have tried so far are neither at thier capacity, nor do they work to great. mabey there is a real 1000 out there that still is 1000 over time, i have not found it.
i am not a brand monkey, but when it does what the label says , then that is a good start. because the overrated ones worked worse than the properly rated ones.
check out also the enloop at about 800ma as things that use AAA dont usually drain the battery in 20 minutes
also the titanium brand has a psudo enloop.
the only thing i can say for SURE at this point, is energyser, even way back when the capacity was poorer still, STILL have that actual capacity after use.
havent tried the duracell.